Microelectronic devices, such as semiconductor devices, field emission displays, read/write heads, and other products that include integrated circuits, are generally fabricated on and/or in microelectronic workpieces using several different types of machines (“tools”). Many such processing machines have a single processing station that performs one or more procedures on the workpieces. In a typical fabrication process, for example, one or more layers of conductive materials are formed on the workpieces during deposition stages. The workpieces are then typically etched and/or planarized (i.e., chemical-mechanical planarization) to remove an “overburden” portion of the deposited conductive layers and thus form electrically isolated contacts and/or interconnect lines.
Plating tools that plate metals or other materials onto the workpieces are becoming an increasingly useful type of processing tool. Electroplating and electroless plating techniques can be used to deposit copper, solder, permalloy, gold, silver, platinum, polymeric materials and other materials onto workpieces for forming blanket layers or patterned layers. A typical copper plating process involves depositing a barrier layer on the workpiece that conforms to micro-recesses and other features and then depositing a copper seed layer onto the barrier layer using chemical vapor deposition (CVD), physical vapor deposition (PVD), electroless plating, or other suitable deposition processes. After forming the seed layer, a blanket layer or patterned layer of copper is plated onto the workpiece by applying an appropriate electrical potential between the seed layer and another electrode in the presence of an electroprocessing solution (i.e., an acidic electrolyte). The workpiece is then cleaned, etched, and/or annealed in subsequent procedures before transferring the workpiece to other processing machines.
FIG. 1 schematically illustrates an embodiment of a single-wafer processing station 1 that includes a container 2 for receiving a flow of electroplating solution from a fluid inlet 3 at a lower portion of the container 2. The processing station 1 can include an anode 4, a plate-type diffuser 6 having a plurality of apertures 7, and a workpiece holder 9 for carrying a workpiece 5. The workpiece holder 9 can include a plurality of electrical contacts arranged to circumscribe a first diameter. The contacts of the workpiece holder 9 contact a perimeter portion of the workpiece for providing electrical current to the seed layer on the surface of the workpiece 5. When the seed layer is biased with a negative potential relative to the anode 4, it acts as a cathode. In operation, the electroplating fluid flows around the anode 4, through the apertures 7 in the diffuser 6, and against the plating surface of the workpiece 5. The electroplating solution is typically an acidic electrolyte that conducts electrical current between the anode 4 and the cathodic seed layer on the surface of the workpiece 5. Therefore, ions in the electroplating solution plate the surface of the workpiece 5.
The plating machines used in fabricating microelectronic devices must meet many specific performance criteria. For example, many processes must be able to form small contacts in submicron recesses, such as vias that are less than 0.5 micron wide and are desirably on the order of 0.1 micron wide. The plated metal layers should also be of a uniform thickness across the surface of the workpiece 5. One factor that influences the uniformity of the plated layer, and especially the integrity of the plated material in the submicron micro-recesses, is the current density across the surface of the workpiece.
Another objective of electrochemical deposition processes according to the prior art is to maximize the real estate available for forming integrated circuits on the workpiece. Existing contact assemblies typically include a plurality of fingers that project radially inwardly from a ring. Each of the fingers includes a contact point, and the contact points are typically arranged to circumscribe a circle with a slightly small diameter than the workpiece. To maximize the available real estate for forming integrated circuits, the diameter circumscribed by the contact points is typically selected to be as close to the perimeter edge of the workpiece as possible. Therefore, a significant number of tool manufacturers have expended significant resources to develop contact rings that minimize the distance that the contacts extend radially inwardly from the perimeter edge of the workpiece.
Although electrochemical deposition processes are widely used in semiconductor fabrication applications, it is becoming difficult to form uniform layers that completely fill the submicron micro-recesses. One factor contributing to the difficulty of electrochemical deposition processes is that very thin seed layers are necessary to fill 0.1–0.5 micron recesses. The ultrathin seed layers are typically discontinuous layers of copper that do not uniformly cover the topography of the workpieces. As a result, an IR drop occurs across thin seed layers, and the amount of copper that each contact engages varies across the workpiece. The IR drop is exacerbated because oxidation greatly impairs the conductivity of the copper seed layer. Moreover, acidic electroplating baths momentarily etch the copper seed layer before an electrical current is established in the bath causing a further reduction of conductivity. Thus, reduced conductivity of the copper seed layer further increases the IR drop.
The IR drop across the seed layer and the non-uniformities of ultrathin seed layers having a thickness of between 100–1000 Å cause a non-uniform current distribution in which the electrical current at the center of the workpiece is less than the current at the perimeter for an initial portion of the plating cycle. This produces non-uniform surfaces across the workpiece and voids within the submicron micro-recesses. Therefore, the semiconductor industry is currently seeking to reduce such non-uniformities and voids associated with electrochemical deposition processes.